This invention relates generally to light-emitting devices and, more specifically, to a class of semiconductor diodes known as superluminescent diodes. Light-emitting diodes are well known semiconductor devices in which an electrical current is passed through a diode junction and produces light emission in an active layer of semiconductor material at the junction. At least one facet of the device is coated with an anti-reflective material, through which light is emitted. This is to be contrasted with a laser diode, in which stimulated emission of light occurs, and there are usually two opposed reflective facets. There are repeated reflections of light between the facets before a coherent laser beam emerges. The resulting laser beam usually has a very narrow spectral width, i.e. it is practically monochromatic.
Light-emitting diodes operating at relatively high powers and having a relatively broad spectral width are called superluminescent light-emitting diodes. There is a need for these devices in fiber optic systems having a requirement for low Rayleigh backscattering, such as in fiber optic gyroscopes, or for low madal noise. Commercially available superluminescent light-emitting diodes emit light at powers as high as 4-6 mW (milliwatts). However, when the power in these devices is increased above about 1-2 mW, the frequency spectrum is substantially narrowed. Driving the devices to higher powers may eventually cause lasing, in spite of the presence of the anti-reflective coating, since even the best anti-reflective coating will reflect some proportion of the light impinging on it, and lasing will eventually occur if the power is increased to a high enough level. The lasing threshold for pulsed diode operation increases with decreased facet reflectivity. The only successful high-power anti-reflective coated superluminscent diodes were made by dynamically monitoring the pulsed laser threshold during the coating process. For this reason, the anti-reflective coatings in superluminescent light-emitting diodes have to be carefully controlled to permit operation at higher power levels.
When a superluminescent diode having one or both facets coated with an anti-reflective material is operated at a high enough current, the spectral content of the output light may still cover a desirably broad band of wavelengths. However, above a certain power level the device operates more and more like a laser, and its output spectrum is characterized by narrow modal lines spread over a relatively broad band. In this lasing mode of operation, the device is said to operate with a high degree of Fabry-Perot modulation, the characteristic laser cavity modulation that is undesirable for applications like the fiber optic gyroscope. These applications require very low Rayleigh backscattering noise, which can only be obtained with a low coherence length and a wide spectral width.
As the power of a superluminescent light emitting diode is increased and its spectral width is consequently decreased, the coherence length of light from the device is increased. The coherence length is another measure of the spectral purity of light, and is inversely proportional to spectral width. As the spectral width becomes narrower, the coherence length increases. Moreover, if the device operates with a large degree of Fabry-Perot modulation and moves into a lasing mode, the coherence length is inversely proportional to the spectral width of the individual modal lines in the intensity-wavelength characteristic of the device. Thus, the coherence length for the lasing mode of operation is several orders of magnitude larger than the coherence length for a superluminescent diode.
The requirement for a light-emitting device with low coherence length and relatively high power is simply not attainable with presently available superluminescent diodes using antireflective coatings to suppress lasing. The cross-referenced U.S. Pat. No. 4,634,928 proposes one technique for the suppression of lasing in a light-emitting device. Basically, that approach employs means within the semiconductor structure for producing a non-uniform gain profile along the active layer of the device. The non-uniformity of the gain profile results in a broadening of the frequency spectrum of emitted light. As the power is increased, the spectral width increases even more, permitting the output of relatively high powers while maintaining a broad spectral width. The present invention relates to an alternate approach for the suppression of lasing in high-power light-emitting diodes.
Some years ago, D. R. Scifres et al. reported in the IEEE Journal of Quantum Electronics, QE-14, 223 (1978), experimenting with a different type of structure that showed promise as a superluminescent diode. Conventionally, a semiconductor laser is constructed to lase in a direction normal to the crystalline cleavage plane along which the facets are formed. These researchers constructed a laser at an angle inclined to the normal direction, such that light propagating at an internal angle of zero, i.e. parallel to the longitudinal direction of the laser, would impinge on the facets at a small angle to the perpendicular. The Scifres et al. structure was of the "gain-guided" type. All light-emitting semiconductors emit light from a diode junction to which power is supplied from a contact stripe formed on the device. If a narrow electrical contact is employed to supply the current, lasing action will be limited to a correspondingly narrow region, with the lateral waveguide boundary defined by the gain profile, i.e. with no intentional refractive index profile built into the structure. This process is generally referred to as gain guiding.
The Scifres et al. device was run in a pulsed mode and, although superluminescence was observed, a large proportion of the output was due to lasing. Moreover, there was an observed tendency at higher currents for the internal beam angle to move toward zero, which minimizes reflectivity losses at the facets and pushes the device more strongly into lasing operation.
It will be appreciated from the foregoing that there is still a need for a superluminescent diode with the characteristics of high power, large spectral width and low Fabry-Perot modulation. Specifically, the requirement is for a device operable at powers well in excess of 10 mW (milliwatts), a spectral half-width of at least 50 Angstroms, and at most 10% Fabry-Perot modulation. The present invention meets or exceeds these requirements without difficulty.